Pipe-laying

ABSTRACT

A pipe-support roller assembly (5) for a laybarge (1) has the rollers (11) resiliently mounted (15) so that, if an irregularity in the pipe (6) causes a change in the stress on the pipe as it passes over the rollers, the rollers can move so as to accommodate the irregularity and reduce the change in the stress.

This invention relates to the laying of pipelines under water, andespecially to a method and an apparatus for controlling the stress andstrain experienced by a pipe while it is being laid.

One method of installing submarine pipelines for use, for example, inthe oil industry, is to form the pipeline on the deck of a vessel knownas a laybarge, by welding together lengths of previously prepared pipe.The laybarge moves forward continuously, and as it does so the pipelineis fed into the sea from the rear of the laybarge. As the pipeline isfed into the sea, further lengths of pipe are welded onto the end of thepipeline that remains on the laybarge. The pipeline may be up to about1.8 meters in diameter, and typically consists of steel pipe clad inconcrete. At the "field joints" where two lengths of pipe are weldedtogether on board the laybarge, there is of course a gap between theconcrete claddings, which is usually filled with pitch or the like flushwith the surface of the concrete.

The pipeline is supported on the laybarge by a number of pipe supportrollers, which allow the pipeline to run freely as it is fed into thesea. The pipeline descends from the laybarge down to the seabed in acurve that is determined by the stiffness of the pipe, the tension onthe pipe (which is controlled by tensioners acting as brakes near thefront end of the series of rollers), the depth of the sea, and the angleat which the pipe leaves the laybarge.

The steeper the angle at which the pipeline leaves the laybarge, themore directly it descends to the seabed, and the smaller the tensionnecessary to maintain a satisfactory path without, in particular, anabrupt downward curve where the pipeline leaves the laybarge.

There are practical limits on the steepness of the path of the pipelineas it extends along the laybarge, and it is therefore known, in order toincrease the angle at which the pipeline leaves the barge, for its pathalong the series of rollers to be appreciably curved.

This curvature results in stress on the pipeline; the greater thecurvature of the pipeline, the greater the stress. As any stress causesstrain in the material that reduces the quality and lifetime of apipeline, pipelayers have recently been required to ensure that thestress on the pipeline does not rise above a specified maximum level. Ithas been proposed to calculate the optimum curvature for a particularpipe-laying operation so as to minimise the stress and strain on thepipe, and to adjust the exact vertical positions of the pipe-supportrollers, before pipe-laying begins, in order to give the calculatedpath.

The calculated stress and strain analysis assumes that the pipeline hasa smooth surface and a uniform cross-section and stiffness. In practice,however, the pipeline is not uniform. For example, irregularities ashigh as several cm on a 1 meter diameter pipe may occur in the surfaceof the concrete cladding of the pipeline, field joints may not beperfectly flush, integral cylinder buckle arresters or anodes mayproject from the surface of the pipe, or the pipe may be out ofroundness.

When such a local imperfection passes over a pipe support roller, itresults in a sudden alteration in the load distribution on the pipesupport rollers, and consequently may result in a sudden increase in thestress and strain on the pipe at the point of the imperfection. Forexample, when a section of pipeline having a greater diameter than thatused when calculating the optimum heights of the pipe support rollerspasses over a pipe support roller, it will result in a sharp increase inthe stress on the pipe at the point where it passes over that roller,and a sharp decrease in the stress at the rollers adjacent to thatroller.

Even if the pipe is in fact uniform, the movement of the laybarge underthe action of wind and waves may cause changes in the load distributionon the pipe-support rollers and, in the worst case, the pipeline mayactually lift off the last roller or the last few rollers and slam downagain with a sudden stress and strain on the pipe resulting.

Such sharp increases in the load on any one pipe support roller, andthus in the stress and strain on the pipeline, have traditionally beenaccounted for by introducing a "design factor", limiting the radius ofcurvature such that the maximum calculated bending stress in thepipeline is only a proportion of the maximum stress that is actuallyacceptable for the pipeline in question.

The invention is based on the realisation that it is possible to absorbat least part of the transient stresses on the pipe, by constructing thepipe-support rollers so that the height of an individual roller or groupof rollers can change while the pipeline is being laid.

The invention provides a pipe-support roller assembly for a pipe-layingvessel, comprising: one or more rollers arranged in use to support apipe being laid; and a suspension system for mounting the roller orrollers on the vessel; which suspension system is arranged in use tocause or permit movement of the roller, or movement in the samedirection of all of the rollers, mounted on it so as to tend to reducechanges in the total load on all of the rollers mounted on thatsuspension system.

The invention also provides a pipe-laying vessel comprising at least oneroller assembly according to the invention for supporting a pipe that isbeing laid.

The invention also provides a method of laying pipelines from a vessel,in which the pipeline is fed over one or more rollers mounted on asuspension system, and in which that suspension system causes or permitsmovement of the roller, or movement in the same direction of all of therollers, mounted on that suspension system so as to tend to reducechanges in the total load in all of the rollers mounted on thatsuspension system.

By reducing the transient stresses on the pipe as it is laid, it ispossible to lay a pipeline that has been less strained, and consequentlyis of better quality and has a longer service life. Instead, because thedesign factor that allowed for transient stresses can be decreased, apipe of the same quality as previously may be laid while allowing othersources of stress and strain to be higher than was previously possible.For example, by increasing either the curvature of the pipe (and thusthe angle at which it leaves the laybarge) or the longitudinal tension,the same pipe can be laid by the same laybarge in deeper water than waspreviously possible, or a heavier pipe can be laid in the same depth ofwater. Instead, because the effect of weather on the pipeline has beenreduced, the same pipeline can be laid in worse weather than waspreviously possible, increasing the number of days in the year when thelaybarge can be at work in any given water. It will be appreciated thattwo or more of those possibilities may be combined as appropriate.

The invention also makes possible a more accurate prediction of thestress on the pipe, and more accurate monitoring of the actual stress.

The suspension system advantageously comprises means responsive tochanges in the load on the one or more rollers. That means preferablycomprises resilient means, but an actively driven suspension may be usedinstead.

By way of example, an embodiment of the invention will now be describedwith reference to the accompanying drawings, in which:

FIG. 1 shows a side view of a laybarge installing a submarine pipeline;

FIG. 2 shows a schematic side view of a laybarge such as that shown inFIG. 1, to a larger scale than FIG. 1 and showing the roller system;

FIG. 3a shows a side view of one form of roller assembly according tothe invention with air suspension at mean level and preset height atminimum value;

FIG. 3b shows a rear view of the roller assembly according to FIG. 3a;

FIG. 4 shows a side view of the roller assembly according to FIG. 3awith air suspension at maximum level and preset height at maximum value;

FIG. 5 shows a side view of the roller assembly according to FIG. 3awith air suspension at mean level and preset height at maximum value;and

FIG. 6 shows a side view of the apparatus according to FIG. 3a with airsuspension at minimum level and preset height at maximum value.

Referring to the accompanying drawings, and initially to FIGS. 1 and 2,one form of semi-submersible laybarge 1 has two internal ramps 2,3,arranged end-to-end within the length of the laybarge, and one externalramp 4 extending beyond the rear end of the laybarge. The first internalramp 2 is fixed to the laybarge, at an angle of, for example, 9° to thehorizontal (assuming that the laybarge is floating level in the water).The second internal ramp 3 is pivoted to the rear end of the firstinternal ramp 2, and the external ramp 4 is pivoted to the rear end ofthe second internal ramp 3, and each of the latter two ramps is sosupported by means not shown that its angle can be adjusted to a desiredangle for the laying of a particular pipe. Mounted on the ramps 2,3,4are a series of pipe support roller units 5 which support a pipeline 6that is being installed on a seabed 7. Four roller units 5 are fixed tothe first internal ramp 2, four more roller units 5 are fixed to thesecond internal ramp 3, and a final five roller units 5 are fixed to theexternal ramp 4. Also fixed to the first internal ramp are threetensioners 8 which apply a braking force to the pipeline 6. Thetensioners 8 may be of a sort known per se. They consist essentially offriction brake shoes and means for pressing the brake shoes against thesurface of the pipe with a controlled pressure. At the front end of thefirst internal ramp 2 is a region 9 at which the pipeline is assembled,by welding onto the end of it sections of pipe that are carried on thelaybarge for the purpose. The assembly region and the assembly operationmay be of a nature known per se, and in the interests of conciseness arenot further described here.

Referring now to FIGS. 3a to 6, the pipe support rollers on the secondinternal ramp 3 and on the external ramp 4 are mounted in pipe supportroller units 5 each of which comprises two pipe support bogies 10arranged one in front of the other. Each bogie 10 has two pairs offreely rotatable pipe support rollers 11 arranged one in front of theother. The rollers 11 of each pair are arranged in a V, with their axesinclining downwards towards each other, so that they not only supportthe pipeline 6 but also guide it laterally. The two bogies 10 of eachunit are mounted by means of respective pairs of pivots 12 on a frame13. The pivot axes of the pivots 12 are perpendicular to the length ofthe pipe and are horizontal relative to the laybarge 1. The frame 13 ismounted on a second frame 14 by means of an air suspension system 15.The air suspension system 15 comprises six pairs of air bag springs 16sandwiched between the two frames 13,14. The air bag springs 16 arepressurised by means (not shown) which may be conventional to supportthe weight of a pipeline lying on the two bogies 10. The first frame 13and the second frame 14 extend in planes that are parallel to oneanother and to the pipeline 6 and are horizontal from side to siderelative to the laybarge 1. The second frame 14 is pivoted by a hinge 17at its rear end to the rear end of a third frame 18. At the front end ofthe third frame 18 is a downwardly-extending limb 19. The lower end ofthe downwardly-extending limb 19 is connected to the front end of thesecond frame 14 by a strut 20 the length of which can be altered by ascrew adjuster 21 operated by a hydraulic motor 22. A load cell 23, tomeasure the load on the second frame 14 and thus on the roller unit 5,is mounted on the downwardly-extending limb 19 of the third frame 18.The apparatus is supported by a pivot 24 at the centre of the thirdframe 18 on a mounting 25 (shown only schematically) that is fixed tothe ramp 3 or 4. The axis of pivoting of the pivot 24 is parallel tothose of the pivots 12. As may be best seen from FIGS. 3a and 4 of thedrawings, if the length of the strut 20 is changed by means of the screwadjuster 21, moving the front ends of the second and third frames 14 and18 closer together or further apart, the third frame 18 can pivot aboutthe pivot 24, so that the second frame 14 can remain parallel to thepipeline 6 while being raised or lowered by an amount equal toapproximately half of the change in length of the strut 20. As shown inFIG. 3a, when the strut 20 is at its shortest length, the second frame14 lies flat on top of the third frame 18. By virtue of the three pivots12,12,24, the roller unit is effectively self-levelling, adjustingitself so that each of the four pairs of rollers 11 carriessubstantially the same load.

In use, the optimum path for the pipeline along the ramps is calculated,and the positions of the adjustable ramps 3,4 and the lengths of thestruts 20 of the roller units 5 are set so that the pipeline 6 willfollow the desired path with the air bag springs 16 of the roller units5 approximately half-way between a fully-compressed and a fully-expandedcondition. The load on each roller unit 5 is calculated, and thepressure in each set of air bag springs 16 is set accordingly.

The pipeline 6 is laid by moving the laybarge 1 slowly forwards, andallowing the pipeline to run along the rollers 11 and off the end of theexternal ramp 4 with a tension controlled by the tensioners 8. While thepipeline 6 is being laid, if, for example, a projection on the surfaceof the pipeline reaches one of the roller units 5, the pipeline willtend to lift as the projection rides over one of the rollers 11. Thatwill cause an immediate increase in the stress on the pipeline 6 and theload on the roller unit 5 in question. The increased load will betransmitted to the air bag springs 16, which will compress, as shown inFIG. 6, absorbing part of the height of the projection andcorrespondingly reducing the increase in stress. Conversely, if anarrowing of the pipeline 6 encounters the rollers 11, the load on theroller unit 5 in question will drop, and the air bag springs 16 willexpand, as shown in FIG. 4, to take up part of the change. In each case,the adjacent roller units will experience a change in load of oppositesign and, if they are also roller units 5 with airbag springs 16, theywill respond accordingly, tending to provide a further degree ofcompensation.

The load cell 23 may be used to monitor the load on the roller unit 5,either to alert the crew so that they can take remedial action if apermitted maximum load is exceeded, or to record the loads experiencedso that the effect on the quality of the pipeline 6 can be reviewedlater, or both.

As an example of suitable dimensions, for a semi-submersible laybarge 1that is approximately 150 meters long and is capable of laying pipelinesup to 1.8 meters (60") in diameter in water up to 130 meters deep, theroller units 5 may be about 8 meters apart along the external ramp 4,the pairs of rollers 11 on each unit may be about 0.8 meters apart, thestrut 20 may be adjustable by about 0.5 meters, and the travel betweenthe fully-extended and fully-compressed positions of the air bag springsmay be about 0.5 meters. Each roller unit 5 may have six pairs of airbag springs 16 each with a rated capacity of 9 tonnes, giving the rollerunit as a whole a rated capacity of about 100 tonnes.

I claim:
 1. A pipe-support roller assembly for a pipe-laying vessel,comprising: one or more rollers arranged in use to support directly apipe being laid; and a suspension system for mounting the roller orrollers on the vessel; which suspension system is arranged in use tocause or permit movement of the roller, or movement in the samedirection of all of the rollers, mounted on it so as to tend to reducechanges in the total load on all of the rollers mounted on thatsuspension system.
 2. A roller assembly as claimed in claim 1, whereinthe suspension system comprises means responsive to changes in the loadon the one or more rollers.
 3. A roller assembly as claimed in claim 2,wherein the means responsive to changes in the load on the one or morerollers comprises resilient means.
 4. A roller assembly as claimed inclaim 3, wherein the resilience is provided by compression and expansionof a gas.
 5. A roller assembly as claimed in claim 4, wherein thesuspension system comprises at least one airbag spring.
 6. A rollerassembly as claimed in claim 5, comprising means for adjusting the forceexerted by the resilient means to support the pipe.
 7. A roller assemblyas claimed in claim 6 when dependent upon claim 4 or claim 5, whereinthe pressure or volume of gas in the resilient means is adjustable.
 8. Aroller assembly as claimed in claim 1, comprising means for presettingthe height of the one or more pipe support rollers.
 9. A roller assemblyas claimed in claim 8, wherein the height adjusting means comprisesmeans for adjusting the height between a first member arranged to befixed to the vessel and a second member, and the said means that isarranged in use to cause or permit movement of the roller or rollersacts between the second member and the roller or rollers.
 10. A rollerassembly as claimed in claim 9, wherein the height adjusting meanscomprises a third member that is pivotally attached to the first memberand to the second member, and means for adjusting the angle of the thirdmember.
 11. A roller assembly as claimed in claim 10, wherein the angleadjusting means comprises means, preferably a strut of adjustablelength, for adjusting the distance between portions of the second andthird members remote from their point of pivotal attachment.
 12. Aroller assembly as claimed in claim 1, comprising two said suspensionsystems spaced apart in the longitudinal direction of the pipe andmounted on a common supporting member that is pivotably mounted topermit equalisation of the loads on the rollers mounted thereon.
 13. Aroller assembly as claimed in claim 1, wherein at least one saidsuspension system comprises two rollers or groups of rollers spacedapart in the longitudinal direction of the pipe and mounted on a commonsupporting member that is pivotally mounted to permit equalisation ofthe loads on the two rollers or groups of rollers.
 14. A vessel forlaying pipelines that comprises at least one roller assembly as claimedin claim 1 for supporting a pipeline being laid.
 15. A method of layingpipelines from a vessel, comprising the steps of:feeding a pipeline overone or more rollers mounted on a suspension system, and causing orpermitting movement of the roller, or movement in the same direction ofall of the rollers, mounted on that suspension system so as to tend toreduce changes in the total load in all of the rollers mounted on thatsuspension system.